:mod:!faulthandler --- Dump the Python traceback

.. module:: faulthandler :synopsis: Dump the Python traceback.

.. versionadded:: 3.3

This module contains functions to dump Python tracebacks explicitly, on a fault, after a timeout, or on a user signal. Call :func:faulthandler.enable to install fault handlers for the :const:~signal.SIGSEGV, :const:~signal.SIGFPE, :const:~signal.SIGABRT, :const:~signal.SIGBUS, and :const:~signal.SIGILL signals. You can also enable them at startup by setting the :envvar:PYTHONFAULTHANDLER environment variable or by using the :option:-X faulthandler command line option.

The fault handler is compatible with system fault handlers like Apport or the Windows fault handler. The module uses an alternative stack for signal handlers if the :c:func:!sigaltstack function is available. This allows it to dump the traceback even on a stack overflow.

The fault handler is called on catastrophic cases and therefore can only use signal-safe functions (e.g. it cannot allocate memory on the heap). Because of this limitation traceback dumping is minimal compared to normal Python tracebacks:

• Only ASCII is supported. The backslashreplace error handler is used on encoding.
• Each string is limited to 500 characters.
• Only the filename, the function name and the line number are displayed. (no source code)
• It is limited to 100 frames and 100 threads.
• The order is reversed: the most recent call is shown first.

By default, the Python traceback is written to :data:sys.stderr. To see tracebacks, applications must be run in the terminal. A log file can alternatively be passed to :func:faulthandler.enable.

The module is implemented in C, so tracebacks can be dumped on a crash or when Python is deadlocked.

The :ref:Python Development Mode <devmode> calls :func:faulthandler.enable at Python startup.

.. seealso::

Module :mod:pdb Interactive source code debugger for Python programs.

Module :mod:traceback Standard interface to extract, format and print stack traces of Python programs.

Dumping the traceback

.. function:: dump_traceback(file=sys.stderr, all_threads=True)

Dump the tracebacks of all threads into file. If all_threads is False, dump only the current thread.

.. seealso:: :func:traceback.print_tb, which can be used to print a traceback object.

.. versionchanged:: 3.5 Added support for passing file descriptor to this function.

Dumping the C stack

.. versionadded:: 3.14

.. function:: dump_c_stack(file=sys.stderr)

Dump the C stack trace of the current thread into file.

If the Python build does not support it or the operating system does not provide a stack trace, then this prints an error in place of a dumped C stack.

.. _c-stack-compatibility:

C Stack Compatibility

If the system does not support the C-level :manpage:backtrace(3) or :manpage:dladdr1(3), then C stack dumps will not work. An error will be printed instead of the stack.

Additionally, some compilers do not support :term:CPython's <CPython> implementation of C stack dumps. As a result, a different error may be printed instead of the stack, even if the operating system supports dumping stacks.

.. note::

Dumping C stacks can be arbitrarily slow, depending on the DWARF level of the binaries in the call stack.

Fault handler state

.. function:: enable(file=sys.stderr, all_threads=True, c_stack=True)

Enable the fault handler: install handlers for the :const:~signal.SIGSEGV, :const:~signal.SIGFPE, :const:~signal.SIGABRT, :const:~signal.SIGBUS and :const:~signal.SIGILL signals to dump the Python traceback. If all_threads is True, produce tracebacks for every running thread. Otherwise, dump only the current thread.

The file must be kept open until the fault handler is disabled: see :ref:issue with file descriptors <faulthandler-fd>.

If c_stack is True, then the C stack trace is printed after the Python traceback, unless the system does not support it. See :func:dump_c_stack for more information on compatibility.

.. versionchanged:: 3.5 Added support for passing file descriptor to this function.

.. versionchanged:: 3.6 On Windows, a handler for Windows exception is also installed.

.. versionchanged:: 3.10 The dump now mentions if a garbage collector collection is running if all_threads is true.

.. versionchanged:: 3.14 Only the current thread is dumped if the :term:GIL is disabled to prevent the risk of data races.

.. versionchanged:: 3.14 The dump now displays the C stack trace if c_stack is true.

.. function:: disable()

Disable the fault handler: uninstall the signal handlers installed by :func:enable.

.. function:: is_enabled()

Check if the fault handler is enabled.

Dumping the tracebacks after a timeout

.. function:: dump_traceback_later(timeout, repeat=False, file=sys.stderr, exit=False)

Dump the tracebacks of all threads, after a timeout of timeout seconds, or every timeout seconds if repeat is True. If exit is True, call :c:func:!_exit with status=1 after dumping the tracebacks. (Note :c:func:!_exit exits the process immediately, which means it doesn't do any cleanup like flushing file buffers.) If the function is called twice, the new call replaces previous parameters and resets the timeout. The timer has a sub-second resolution.

The file must be kept open until the traceback is dumped or :func:cancel_dump_traceback_later is called: see :ref:issue with file descriptors <faulthandler-fd>.

This function is implemented using a watchdog thread.

.. versionchanged:: 3.5 Added support for passing file descriptor to this function.

.. versionchanged:: 3.7 This function is now always available.

.. function:: cancel_dump_traceback_later()

Cancel the last call to :func:dump_traceback_later.

Dumping the traceback on a user signal

.. function:: register(signum, file=sys.stderr, all_threads=True, chain=False)

Register a user signal: install a handler for the signum signal to dump the traceback of all threads, or of the current thread if all_threads is False, into file. Call the previous handler if chain is True.

The file must be kept open until the signal is unregistered by :func:unregister: see :ref:issue with file descriptors <faulthandler-fd>.

Not available on Windows.

.. versionchanged:: 3.5 Added support for passing file descriptor to this function.

.. function:: unregister(signum)

Unregister a user signal: uninstall the handler of the signum signal installed by :func:register. Return True if the signal was registered, False otherwise.

Not available on Windows.

.. _faulthandler-fd:

Issue with file descriptors

:func:enable, :func:dump_traceback_later and :func:register keep the file descriptor of their file argument. If the file is closed and its file descriptor is reused by a new file, or if :func:os.dup2 is used to replace the file descriptor, the traceback will be written into a different file. Call these functions again each time that the file is replaced.

Example

Example of a segmentation fault on Linux with and without enabling the fault handler:

.. code-block:: shell-session

$ python -c "import ctypes; ctypes.string_at(0)"
Segmentation fault

$ python -q -X faulthandler
>>> import ctypes
>>> ctypes.string_at(0)
Fatal Python error: Segmentation fault

Current thread 0x00007fb899f39700 (most recent call first):
  File "/opt/python/Lib/ctypes/__init__.py", line 486 in string_at
  File "<stdin>", line 1 in <module>

Current thread's C stack trace (most recent call first):
  Binary file "/opt/python/python", at _Py_DumpStack+0x42 [0x5b27f7d7147e]
  Binary file "/opt/python/python", at +0x32dcbd [0x5b27f7d85cbd]
  Binary file "/opt/python/python", at +0x32df8a [0x5b27f7d85f8a]
  Binary file "/usr/lib/libc.so.6", at +0x3def0 [0x77b73226bef0]
  Binary file "/usr/lib/libc.so.6", at +0x17ef9c [0x77b7323acf9c]
  Binary file "/opt/python/build/lib.linux-x86_64-3.15/_ctypes.cpython-315d-x86_64-linux-gnu.so", at +0xcdf6 [0x77b7315dddf6]
  Binary file "/usr/lib/libffi.so.8", at +0x7976 [0x77b73158f976]
  Binary file "/usr/lib/libffi.so.8", at +0x413c [0x77b73158c13c]
  Binary file "/usr/lib/libffi.so.8", at ffi_call+0x12e [0x77b73158ef0e]
  Binary file "/opt/python/build/lib.linux-x86_64-3.15/_ctypes.cpython-315d-x86_64-linux-gnu.so", at +0x15a33 [0x77b7315e6a33]
  Binary file "/opt/python/build/lib.linux-x86_64-3.15/_ctypes.cpython-315d-x86_64-linux-gnu.so", at +0x164fa [0x77b7315e74fa]
  Binary file "/opt/python/build/lib.linux-x86_64-3.15/_ctypes.cpython-315d-x86_64-linux-gnu.so", at +0xc624 [0x77b7315dd624]
  Binary file "/opt/python/python", at _PyObject_MakeTpCall+0xce [0x5b27f7b73883]
  Binary file "/opt/python/python", at +0x11bab6 [0x5b27f7b73ab6]
  Binary file "/opt/python/python", at PyObject_Vectorcall+0x23 [0x5b27f7b73b04]
  Binary file "/opt/python/python", at _PyEval_EvalFrameDefault+0x490c [0x5b27f7cbb302]
  Binary file "/opt/python/python", at +0x2818e6 [0x5b27f7cd98e6]
  Binary file "/opt/python/python", at +0x281aab [0x5b27f7cd9aab]
  Binary file "/opt/python/python", at PyEval_EvalCode+0xc5 [0x5b27f7cd9ba3]
  Binary file "/opt/python/python", at +0x255957 [0x5b27f7cad957]
  Binary file "/opt/python/python", at +0x255ab4 [0x5b27f7cadab4]
  Binary file "/opt/python/python", at _PyEval_EvalFrameDefault+0x6c3e [0x5b27f7cbd634]
  Binary file "/opt/python/python", at +0x2818e6 [0x5b27f7cd98e6]
  Binary file "/opt/python/python", at +0x281aab [0x5b27f7cd9aab]
  Binary file "/opt/python/python", at +0x11b6e1 [0x5b27f7b736e1]
  Binary file "/opt/python/python", at +0x11d348 [0x5b27f7b75348]
  Binary file "/opt/python/python", at +0x11d626 [0x5b27f7b75626]
  Binary file "/opt/python/python", at PyObject_Call+0x20 [0x5b27f7b7565e]
  Binary file "/opt/python/python", at +0x32a67a [0x5b27f7d8267a]
  Binary file "/opt/python/python", at +0x32a7f8 [0x5b27f7d827f8]
  Binary file "/opt/python/python", at +0x32ac1b [0x5b27f7d82c1b]
  Binary file "/opt/python/python", at Py_RunMain+0x31 [0x5b27f7d82ebe]
  <truncated rest of calls>
Segmentation fault